Apparatus for producing electrical conductor films by explosive evaporation



y 1964 E. G. WEBER APPARATUS FOR PRODUCING ELECTRICAL CONDUCTOR FILMS BYEXPLOSIVE EVAPORATION- Filed Jan. 3, 1961 INVENTOR .ZjPW/A/ G. WEBER III; l l ilmfi WM %;g

United States PatentO APPARATUS FOR PRODUCTN G ELECTRICAL CON- DUCTORFILMS BY EXPLOSIVE EVAPORATION Erwin G. Weber, Hopkins, Minn, assignorto Sperry Rand Corporation, New York, N.Y., a corporation of DelawareFiled Jan. 3, 1961, Ser. No. 80,301 Claims. (Cl. 118--49.1)

This invention relates to the deposition of thin films.

The invention concerns itself with both the method of depositing filmsand apparatus for accomplishing the method. The prior art is repleatwith ways of effecting thin films, for example as in the Rubens Patent2,900,282. In that patent reference is made to the vacuum deposition ofthin magnetic films with evaporation being effected by inductively orotherwise heating ferromagnetic material and causing its vapor tocondense onto a substrate. In the present application magnetic ornonmagnetic films may be deposited in vacuum onto a substrate byexplosive- 1y evaporating a conductor which is preferably resistivelypreheated.

Explosive evaporation offers the advantages of: (1) a short depositiontime of less than 1 p.560. which reduces the interaction between themetal vapor and the residual atmosphere in the vacuum to a minimum; (2)absence of contact between the material to be evaporated and othermaterials at high temperature (for example, crucibles, boats, etc.)prevents changes of the material or of the vaporization process; (3)rapid heat generation prevents fractional distillation; (4) shortdeposition time minimizes interaction of the condensing Vapor with thesubstrate (diffusion, chemical reactions, etc.); and (5) exactcalibration of the evaporated total mass necessary is easily carried outby a simple length measurement of the conductor exploded.

It is therefore the primary object of this invention to provide a methodand apparatus for effecting explosive evaporation of an electricalconductor.

Another object of this invention in conjunction with the foregoingobject is the provision of preheating the conductor to supply a part ofthe energy required to vaporize the conductor completely.

Other objects and advantages of this invention will become apparent tothose of ordinary skill in the art after reading the followingdisclosure and claims in view of the accompanying drawing which ispartially schematic.

Novel apparatus for accomplishing the method of this invention is shownin the drawing. Glass bell jar along with its base 12, in which aconvenient outlet (not shown) may be disposed, form means for creating avacuum which is preferably between 1 l0 mm. and -l l0 mm. Hg. Themaximum pressure should not exceed 1X 10* mm. Hg and there is no minimumpressure limit. The solid electrical conductor 14 to be explosivelyevaporated is vertically held at its ends between two high voltageconnector balls 16, 18. The upper ball 16 is electrically connected byconductor 20 which feeds through the'top of the bell jar to a highvoltage terminal 22. The other high voltage terminal 24 is connected byconductor 26 directly to base plate 12, which forms a common ground forthe electrical circuit of the apparatus.

The other internal high voltage ball 18 is electrically connected to arigid conductor 28 which together normally hang free of electrical ormechanical connection, i.e., with the lower end of conductor 28 spacedfrom each of the conductive clamp blocks 30 and 32. Block 30 is securedto base plate 12 as by bolt 34, but block 32 is movable from itsillustrated position toward block 30, the two blocks being biased towardeach other by spring 36. On the right side of block 32, is anotherspring 38 which 'ice is serially connected with a fuse wire 40 to theterminal post 42 which insulatively passes through base plate 12.

Externally, terminal 42 is serially connected to the base plate byswitch 44 and battery 46. Whenever switch 44 is closed, battery 46provides current via post 42 to fuse 40, thence through spring 38, block32, and thence to base plate 12 directly if that block is not insulatedfrom the base plate, or through spring 36 and block 30 if it isinsulated therefrom. This current causes fuse 40 to blow so that thebias of spring 36 pulls block 32 leftwardly to cause conductor 28 to besecurely held between the two blocks. Instead of effecting clamping ofthe conductor 28 in this manner at the beginning of a deposition cycle,conductor 28 may be permanently secured to block 30 or directly to baseplate 12 initially. With the apparatus as shown, the conductor to beexploded need not be drawn tight in connecting it to the balls 16 and18, but instead the lower holder comprised of ball 18 and conductor 28is sufficiently weighty to maintain conductor 14 straight and provide toit any desired tension or tautness. On the other hand, conductor 14 maybe curved or slightly bent if desired even after the bell jar is pulleddown tight against its base.

The next step in the process is to preheat conductor 14 resistively.This is accomplished in the illustrated apparatus by closing switch 48so that current from battery 50 will proceed through terminal post 52which is insulated from base plate 12, thence through the upstandingrelatively rigid conductor 54, conductor 56 to the fork-shaped connector58. The V between the tongs 60 and 62 electrically connects to conductor20, and consequently to ball 16, conductor 14, and the lower conductorholder 18, 28, completing the circuit to the base plate 12. Asillustrated, one tong 62 of connector 58 is serially connected by spring64 and a second fuse wire 66 to a hook 68 which is electricallyconnected to a second upstanding relatively rigid conductor 70. In thismanner, connector 58 is disconnectably held in electrical contact withconductor 20, i.e., to one end of conductor 14. With switch 48 closed,resistive heating of conductor 14occurs. Of course, if switch 48 hadbeen closed before fuse 40 blew, no heating of conductor would haveoccurred until conductor 28 Was electrically connected, indicating thenon-necessity of switch 48 to start the heating period runmng.

As soon as the conductor 14 is heated to a desired temperature, whichmay be sensed for example by a pyrometer (not shown), switch 48 may beopened it desired, but preferably the resistive heating is stopped byclosing switch 72. This, as will be seen, effects a disconnecting of thedisconnectable fork 58 from conductor 20, by causing current frombattery 74 to flow via conductors 54, 56, connector 58, spring 64, fusewire 66, and conductor 70 which is insulatively returned through baseplate '12 to'the terminal post 76, completing the connection back toswitch 72. This current is in excess of that which the fuse 66 cancarry, so its blows. At this instant, since the conductor 56 is biaseddownwardly by spring 78, the forkshaped connector 58 is pulled away fromconductor 20 so that even if switch 48 is still closed, the current frombattery 50 cannot be returned through conductor 14. As soon as it isnoted that the connector 58 is moved out of contact with conductor 20,the double-pole double-throw switch is thrown from its leftward(illustrated) position to the right so as to provide power from the voltterminals 81 to energize relay solenoid 82 causing its switch contact 84to close. (Preferably, this relay is in an oil bath as indicated.) Thisimmediately discharges condenser 86 and provides across conductor 14 avoltage which is sufiiciently strong to explosively evaporate conductor14.

The high voltage obtainable from the discharge of condenser 86, whichmay be a bank of condensers totaling 6 ,uf. capacity for example, may beeffected by previously charging that condenser in any desired manner,for exam ple as by the 110 volt source 81 while switch 80 is in itsillustrated position. This energizes the charging source 88, andcondenser 86 is charged via a high resistance (9M, for example) resistor90.

When conductor 14 explodes upon the application of the high voltage toterminals 22, 24, the vapor deposits itself on a masked substrate 92. Asillustrated the masking effects a plurality of circular apertures 94,for example 8 mm. diameter, so the resulting films deposited throughthese apertures are circular, but the masking apertures may be of anyconfiguration desired. Alternatively, the substrate need not be maskedunless desired. The number of apertures in the mask may of course be anynumber. Without the additional masking of the cold ends of conductor 14as effected by masks 96, the films are likely to be heavily perforateddue to splattering caused by the ends of conductor 14 being coldrelative to the center of the conductor. Masks 96 inhibit thissplattering. These masks may take any desired configuration, and may beglass sleeves disposed about the cold ends.

Conductor 14 may be any desired type of electrical conductor; it may beeither magnetic or non-magnetic, to cause either magnetic ornon-magnetic films respectively. For example, platinum wire(non-magnetic) may be used to effect platinum films. In the magneticcategory, conductor 14 may be any magnetic type of wire, for example,one of the so-called Permalloys, which as is well known, include nickeland iron. Perminvar in any of its forms, as well as all other types ofbinary, ternary, etc., alloys, or non-alloyed magnetic materials may beemployed to obtain the type of films desired. One highly important useof this invention is the deposition of one or more metal films of themagnetic type and employment thereof in any device requiring informationstorage capabilities such as in logic devices and computers generally.For this purpose, conductor 14 is preferably a Permalloy wire, forexample of the 80% nickel, balance iron type. More preferably, in orderto obtain films which are substantially non-magnetostrictive, the wirehas a composition of 81% nickel, remainder iron.

For certain materials, it is desirable that the preheating temperatureof conductor 14 cannot be so high as to melt the conductor, thoughpreferably the preheating temperature is adjacent the melting point ofthe conductor. The purpose of preheating the conductor is to supplythereto some of the energy which is required to allow the high voltageto completely explosively evaporate the conductor. For an 80-20permalloy wire 4 inches long, 20 mils diameter, the optimal preheatingtemperature of the wire is around 1000 C. At a much lower temperaturethe amount of heat generated in the wire by the discharge current ofcondenser 86 is too small to completely evaporate the wire, and at ahigher temperature the evaporation is non-uniform because of apparentweak spots in surfacetension effects, and apparent reduction inconductivity reducing the current caused heat. In both cases splatteringof the wire may occur in characteristic ways, depending on factors suchas local cross sectional variations or local inhomogeneities. The toocold wire disintegrates into a few big droplets. The too hot wire maydisintegrate into many small droplets. The unavoidable cooler ends ofthe wire always exhibit the too cool splattering and are thereforeshielded by masks 96 as above indicated. With a temperature of around1000 C., the above-mentioned four-inch wire caused films 300-500 A.thick on a substrate one-inch from the wire.

The minimum preheat temperature is related to the total amount of energyprovided by the discharge of condenser 86 and the amount of energyrequired to successfully cause the explosive evaporation. That is, ifthe condenser discharge energy was in excess of the amount of energyrequired to explode a preheated wire, this excess energy could be usedto explosively evaporate a cold wire. The minimum preheat temperaturethen would be that temperature to which the wire must be preheated inorder that sufiicient capacity discharge energy is available tosuccessfully explode the wire. Though the wire may be exploded withoutany preheating, the capacitor discharge energy, provided by thepresently used equipment, alone does not provide sufficient energy tocompletely transform the solid wire to its vapor state. Preheating isrequired to effect complete transformation.

After the desired preheating temperature is reached, it is held for somedesired time, for example ten minutes, to allow outgassing. The limitingfactor with respect to the maximum preheat time is the possibility ofoxidizing the surface of the wire which would change the chemicalcharacteristics of the wire with resultant changes in conductivity.Therefore, the preheating is concluded prior to the occurrence ofsignificant oxidation. Of course, the higher the vacuum, the less thepossibility of oxidation, and the more outgassing that can be effected.The higher the vacuum, the smoother and less structured are thevapor-clouds leaving the wire.

As between a plurality of 8 mm. diameter circular permalloy filmsdeposited onto ordinary glass microscope slides, a remarkable uniformitywith regard to the type of hysteresis loop, coercive force, andanisotropy field was noted. For most films the coercive force was in therange of from 2*l0 oe. and the anisotropy field was always greater than100 oe. Uniformity from evaporating one wire to the next can be expectedif the wires have like characteristics if the preheat temperature is thesame, and switch connects to coil 82 at the same time relative to theclosing of switch 72. As exemplary, this time element may beapproximately 0.1 sec. maximum, and the minimum time is that requiredfor spring 78 to remove fork 58 from the high voltage feed-through 20sufliciently to prevent a spark from jumping between the fork 58 and thefeed-through 20 when the high voltage is applied. If wire 14 coolsslowly after preheating stops or has been heated far in excess of thetemperature required, the maximum time can be extended slightly.

In certain cases it has been found desirable to permit the wire to coolsubsequent to the preheating step and prior to the time that theexplosion operation is initiated. In these cases, a superior outgassingof the material may be achieved, this being due to the elimination ofthe high vapor pressure which would normally be expected to existbetween the inner phase of the heated wire as it is being heated. Forexample, the thermal-conduction of the balls 16 and 18, which have acooling effect upon the end portion of the wire 14, sets up thermalgradients along the length of the wire body. The presence of thegradients along the length of the wire provides a source of integralzones of relatively high vapor pressure along the wire. Thus, thetechnique of preheating to a relatively high temperature followed by acooling from this high temperature can be utilized to eliminate theformation of these zones of high vapor pressure.

Throwing switch 80 rightwardly at a predetermined time after closingswitch 72 may be effected automatically in any of numerous manners.

Preferably, the voltage to which condenser 86 is charged is around 20kv., though it may vary considerably from this voltage, if desired ornecessary in accordance with the characteristics of the particular typeelectrical conductor being exploded. 20 kv. is sufficient to causeexplosion of 81 Permalloy wire which is six-inches long and 25 mils indiameter, or which is four inches by 30 mils diameter. There is noapparent maximum limit for the high voltage and the minimum is thatnecessary to cause the desired explosion of a particular type and sizeof wire used.

While the technique described above has been related to the explosion ofmaterials substantially from the solid phase, it should be pointed outthat the technique is adaptable to the treating of materials which aremaintained in the liquid or substantially liquid phase. Such a techniqueis particularly adaptable for use in connection with materials whichhave a relatively high vapor pressure. In this connection, the liquidphase may be achieved by either reducing the pressure or preheating ofthe solid to a sufficiently high temperature to transform the solid intothe liquid. For certain materials, no specific modification of theequipment embodiment disclosed hereinabove is required inasmuch as thesurface tension of these materials may be sufiiciently high to maintainthe material in a certain desired physical form. For other materials,however, particularly those having a strong tendency to sublime at thelow pressure encountered, it may become necessary to encapsulate orotherwise enclose the substance within a certain enclosure prior toexplod ing the same. Any suitable enclosure may be employed, such as,for example, polyethylene or the like. This encapsulation technique isalso particularly adaptable to use in connection with materials haw'ngunreasonably high vapor pressures, such as, for example, lead, zinc orthe like. In each case, the explosive evaporation is accomplished byfollowing the techniques as set forth hereinabove and there isaccordingly no unusual problem encountered in the explosion operationper se.

Thus, it is apparent that there is provided by this invention a devicein which the various objects and advantages herein set forth aresuccessfully achieved.

Modifications of this invention not described herein will becomeapparent to those of ordinary skill in the art upon reading thisdisclosure. Therefore, it is intended that the material contained in theforegoing description and the accompanying drawing be interpreted asillus trative and not limitative, the scope of the invention beingdefined in the appended claims.

What is claimed is:

1. Apparatus for producing at least one film from an electricalconductor comprising vacuum means, an integral length of electricalconductor disposed within said vacuum means, means inside said vacuummeans for holding said conductor by its respective end portions, asubstrate adjacent said conductor when so held, means for resistivelyheating said conductor, means for applying across the heated conductor avoltage sufficient to explosively evaporate said conductor onto saidsubstrate as said film, and means disposed between said substrate andthe conductor end portions, a part of which are exposed between saidholding means and cold relative to the central length of said conductorwhen resistively heated as aforesaid, to prevent splattering of saidcold ends under such substrate when said conductor is explosivelyevaporated.

2. Apparatus for producing at least one film from an electricalconductor comprising vacuum means, means inside said vacuum means forholding said conductor by its respective end portions, a substrateadjacent said conductor when so held, means for resistively heating saidconductor, and means for applying across the heated conductor a voltagesufiicient to explosively evaporate said conductor onto said substrateas said film, said holding means arranged to hold said conductorvertically and including a lower-end weighting holder which is normallyfree of electrical or mechanical connection to cause said conducter tobe taut, and means inside said vacuum means for causing the saidweighting holder to continue holding said conductor taut and to completeat a predetermined time an electrical connection of the conductor tosaid heating and voltage applying means, said last mentioned meansincluding two clamping means biased toward each other and a fusenormally holding the clamping means apart with a part of said Weightingholder being disposed between said clamping means, and means for causingsaid fuse to blow at said predetermined time to allow clamping of saidweighting holder by said clamping means.

3. Apparatus for producing at least one film from an electricalconductor comprising vacuum means, means inside said vacuum means forholding said conductor by its respective end portions, a substrateadjacent said conductor when so held, means for resistively heating saidconductor, and means for applying across the heated conductor a voltagesufficient to explosively evaporate said conductor onto said substrateas said film, said heating means including disconnectable means insidesaid vacuum means, electrically connected to one end of said conductor,and said apparatus further including means for disconnecting saiddisconnectable means.

4. Apparatus as in claim 3 wherein said disconnecting means includes afuse normally holding said disconnectable means and said one end of theconductor in an electrical connection and further includes means forblowing said fuse before said voltage is applied.

5. Apparatus as in claim 4 wherein said disconnecting means alsoincludes bias means for positively pulling said disconnectable meansaway from its connection to said conductor after said fuse blows.

References Cited in the file of this patent UNITED STATES PATENTS2,976,174 Howard Mar. 21, 1961 FOREIGN PATENTS 702,937 Germany Jan. 23,1941 OTHER REFERENCES Physical Review, vol. 79 (1950), (p. 213 reliedon).

1. APPARATUS FOR PRODUCING AT LEAST ONE FILM FROM AN ELECTRICALCONDUCTOR COMPRISING VACUUM MEANS, AN INTEGRAL LENGTH OF ELECTRICALCONDUCTOR DISPOSED WITHIN SAID VACUUM MEANS, MEANS INSIDE SAID VACUUMMEANS FOR HOLDING SAID CONDUCTOR BY ITS RESPECTIVE END PORTIONS, ASUBSTRATE ADJACENT SAID CONDUCTOR WHEN SO HELD, MEANS FOR RESISTIVELYHEATING SAID CONDUCTOR, MEANS FOR APPLYING ACROSS THE HEATED CONDUCTOR AVOLTAGE SUFFICIENT TO EXPLOSIVELY EVAPORATE SAID CONDUCTOR ONTO SAIDSUBSTRATE AS SAID FILM, AND MEANS DISPOSED BETWEEN SAID SUBSTRATE ANDTHE CONDUCTOR END PORTIONS, A PART OF WHICH ARE EXPOSED BETWEEN SAIDHOLDING MEANS AND COLD RELATIVE TO THE CENTRAL LENGTH OF SAID CONDUCTORWHEN RESISTIVELY HEATED AS AFORESAID, TO PREVENT SPLATTERING OF SAIDCOLD ENDS UNDER SUCH SUBSTRATE WHEN SAID CONDUCTOR IS EXPLOSIVELYEVAPORATED.